1. Field of the Invention
This invention relates to transmitting two signals on one communication channel and more particularly transmitting data during the active video portion of a video signal.
2. Description of the Prior Art
Several techniques have been developed to allow the transmission of two signals on the same communications channel. When the communications channel is a specified bandwidth of the spectra, these methods include time division multiplexing, transmitting orthogonally polarized waves, transmitting by two transparent methods, each transparent to the other and frequency interleaving.
In the case of video signals, such as NTSC and PAL signals, several different methods have been used for transmission of additional information in the band. For example, a chrominance (color) signal is transmitted by frequency interleaving the chrominance signal with the luminance (black and white) signal. In particular, for the NTSC standard, the luminance signal and the chrominance signals are frequency interleaved. Of course, the chrominance signals are closely related to the luminance signal and the signals exhibit a high degree of correlation.
There are also intervals where no picture information is being transmitted such as during the vertical and horizontal blanking intervals. In some systems, for example close captioned television for the heating impaired, the close captioned information is transmitted during the vertical blanking intervals. Of course, the data rate of transmission systems using the blanking period is relatively low, typically about 20,000 bits per second. Although such transmission rates may be suitable for close caption television, this is far too low for the suitable transmission of large volumes of information such as the 1.544 megabits per second for T1 transmission.
Therefore, it is a first object of this invention to permit transmission at a higher data rate than those permitted during the blanking period. It is a second object of this invention to permit transmission at the higher data rate without causing any noticeable interference from the transmitted information to conventional television receivers. It is yet a third object of this invention to achieve such transmission using frequency interleaving.